Waveguide structures

ABSTRACT

A waveguide structure comprising a medium disposed of a sensing layer ( 21 ), a second layer of material ( 22 ) having a refractive index greater than that of the medium, and a substrate ( 24 ). The structure defines a waveguide capable of supporting an optical mode confined in a sensing layer. The medium is adapted for performing chemical or biological reactions within the medium which will result in a change of an optical property of the sensing layer of the waveguide. The thickness and refractive indexes of the layers are chosen such that an optical mode confined in the sensing layer will suffer substantially anti-resonant reflection as a consequence of the interface between the sensing layer and the second layer and the interface between the second layer and the substrate. Alternatively, the waveguide may comprise a low index sensing medium held between a superstrate and a substrate each of which has a refractive index higher than that of the medium. The waveguide may be capable of supporting two modes, such that one of the modes may be used as a reference during measurement of optical properties of a medium. The waveguide may be capable of supporting a leaky waveguide mode, the presence of the leaky waveguide mode being indicated by a peak of light returned from the waveguide.

TECHNICAL FIELD OF THE INVENTION

This invention relates to waveguide structures, and particularly thoughnot exclusively to waveguide structures suitable for use as opticalsensors.

BACKGROUND OF THE INVENTION

Sensors which are capable of monitoring biological interactions in realtime and with high sensitivity are of considerable importance in lifescience research. Several sensors exist which monitor changes in therefractive index (or other parameters) of a biological sample, caused bymolecular interactions. In a typical sensor an evanescent waveassociated with an optical mode existing in a high refractive indexdielectric layer of a waveguide extends into a biological sample, whichis held in a gel. A change of the refractive index of the sample willmodify an optical property of the waveguide mode, and detection of thischange will provide dynamic information relating to interactionsoccurring within the biological sample.

Known optical evanescent sensors include those based on surface plasmonresonance and those based on dielectric waveguiding techniques (see forexample Welford, K (1991) Surface plasmon-polaritons and theiruses—Optical and Quantum Electronics, 23, 1-27; Smith, A. M. (1987)Optical waveguide immunosensors, Proc. SPIE 798 Fibre Optic Sensors II,206-213); R. H. Ritchie, Phys. Rev. 106, 874 (1957).

Sensors which use surface plasmon resonance comprise a thin metal layer(typically a few tens or hundreds of Angstroms thick) deposited onto adielectric prism or grating, and a sensing layer (or a fluid) whoseoptical properties are of interest provided at an opposite surface ofthe metal layer. Measurements are made by directing light via the prismor grating onto that side of the metal layer which is not in contactwith the sensing layer, and detecting light which is reflected from thesame side of the metal layer. A surface plasmon resonance excited by theincident light will result in the absorption of that incident light, anda consequent dip in the reflected light intensity. The condition forexciting a resonance (i.e. the angle of incident light which will excitea resonance) is sensitive to changes in the optical properties of thesensing layer. The optical properties of the sensing layer may bemonitored by detecting changes in the angle of incidence which excites aresonance

The resolution, and hence the sensitivity, of sensors which utilisesurface plasmon resonance is limited by the resonance width (i.e. therange of angles of incident light which will excite resonance). Thiswidth is determined ultimately by the amount of absorption of incidentlight into the metal layer. Absorption is considerable at wavelengthscommonly used for biological measurements, and the maximum resolution ofsurface plasmon sensors is correspondingly restricted.

The angle of incident light which excites a surface plasmon resonancewill alter if the wavelength of the incident light is changed.Variations in the wavelength of incident light will thus introduce anerror into measurements. This is a further limitation of surface plasmonresonance sensors, since wavelength-stabilised sources of incident lightare needed to allow accurate measurement.

A waveguide structure, based upon the surface plasmon resonancestructure and known as a leaky mode waveguide, is described by R. P.Podgorsek, H. Frarke and J. Woods (1998) Monitoring of the Diffusion ofVapour Molecules in Polymer Films using SP-Leaky-Mode Spectroscopy,Sensors and Actuators B-Chemical, Vol.51, No.1-3, pp.146-151. Thewaveguide comprises a substrate, a thin metal layer disposed on top ofthe substrate, and a sensing layer whose optical properties are ofinterest disposed as a further layer on top of the layer of metal. Thesensing layer has optical properties which change if the medium isexposed to conditions to be sensed, and may be for example dextran gel.

The leaky mode excited within the sensing layer is of a type known inthe art as a bulk mode. This contrasts with the mode which is excited bysurface plasmon resonance sensors, which mode is known in the art as asurface mode. Generally only one mode may be excited in surface plasmonsensors (the mode must be a TM mode), whereas the leaky mode waveguideallows the excitation of a series of modes (the modes may be anycombination of TE and TM).

A leaky mode of the waveguide, i.e. a bulk mode which is centred on thesensing layer, is excited by directing light towards the layer of metalor metal alloy through the substrate over a range of incident angles.The presence of an excited leaky mode is determined by detecting theintensity of light returned from the waveguide over a range of angles.When light is coupled to a leaky mode of the waveguide this is seen as adip in the intensity of light emitted from the waveguide. A change of anoptical property of the sensing layer will modify the angle of incidentlight required to excite the leaky mode. The angle at which the dip ofintensity is returned from the waveguide will change accordingly.

The leaky mode waveguide is advantageous compared to surface plasmonresonance because a bulk mode of the waveguide is excited rather than asurface mode. This bulk mode is centred on the sensing layer of thewaveguide and is therefore considerably more sensitive to changes of theoptical properties of the sensing layer than the surface mode providedby surface plasmon resonance.

A disadvantage of known leaky mode waveguides is that detection opticsare required to detect a dip in the intensity of light returned from thewaveguide, and to follow angular movement of that dip. The absence oflight is inherently more difficult to detect than a peak of lightintensity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a leaky mode waveguide whichwill return a peak of intensity when a leaky waveguide mode is excited.

According to a first aspect of the invention there is provided anoptical sensor comprising a waveguide having a substrate, a layer ofmetal or metal alloy disposed on top of the substrate, and a mediumdisposed as a sensing layer on top of the layer of metal or metal alloy,the medium having optical properties which change if the medium isexposed to conditions to be sensed, the sensor further comprising meansfor directing light towards the layer of metal or metal alloy throughthe substrate over a range of incident angles, and detection means fordetecting the intensity of light returned from the waveguide over arange of detection angles, the means for directing light beingconfigured to direct light such that a leaky waveguide mode is excitedwithin the sensing layer, and the means for detecting the intensity oflight being arranged to detect variations with detection angle in theintensity of returned light resulting from the excitation of the leakywaveguide mode; characterised in that the waveguide is configured suchthat the overlap of the optical field with the layer of metal or metalalloy is less for light incident at an angle which results in excitationof a leaky waveguide mode than for light incident at an angle which doesnot result in excitation of a leaky waveguide mode, whereby the detectedintensity peakswat a detection angle related to an incident angle whichresults in excitation of a leaky waveguide mode.

The invention is advantageous because it allows for the easy detectionof a waveguide mode.

The term metal alloy is intended to include mixtures of metals andmixtures of two or more elements which include at least one metal.Metals or metal alloys are used because they have a sufficiently highimaginary part of refractive index that an optical field extending intothe metal or metal alloy suffers significant loss. The term metal ormetal alloy is therefore intended to include any material having animaginary part of refractive index comparable to that of a metal ormetal alloy.

Preferably, the substrate comprises a prism or grating for couplinglight into the waveguide mode.

An optical source comprising a laser, a light emitting diode or a sourcecapable of producing a broad spectrum of wavelengths of light may beused to provide the incident light. The use of a light emitting diode,or a broad band source, is made possible by the relative wavelengthinsensitivity of the waveguide mode of the invention.

The detection means is preferably a charge-coupled-device array (CCD)comprising cells of sufficiently small dimensions to allow resolution ofthe intensity variations resulting from the excitation of the waveguidemode.

The detection means may comprise a single photo-diode which is capableof being translated across the light returned by the waveguide. Bytranslating the photo-diode through a series of positions, thephoto-diode may be made to provide a measurement of intensity at eachposition, thereby giving a measurement similar to that which will beprovided by the CCD array.

The thickness of the layer of medium is preferably greater than 200 nm,and most preferably greater than 300 nm. The layer of medium is requiredto be thicker than that typically used for surface plasmon resonancesensors, in order to support the waveguide mode which is excited withinthe medium.

According to a second aspect of the invention there is provided a methodof optical sensing comprising providing a waveguide comprising asubstrate, a layer of metal or metal alloy disposed on top of thesubstrate, a medium disposed as a sensing layer on top of the layer ofmetal or metal alloy, the medium having optical properties which changeif the medium is exposed to conditions to be sensed, directing lighttowards the layer of metal or metal alloy through the substrate over arange of incident angles, and detecting the intensity of light returnedfrom the waveguide over a range of angles, wherein the incident light isdirected such that a waveguide mode is excited within the sensing layer,and variations in the intensity of returned light resulting from theexcitation of the waveguide mode are detected; characterised in that thewaveguide is configured such that the overlap of the optical field withthe layer of metal or metal alloy is less for light incident at an anglewhich results in excitation of a leaky waveguide mode than for lightincident at an angle which does not result in excitation of a leakywaveguide mode, whereby the detected intensity peaks at a detectionangle related to an incident angle which results in excitation of aleaky waveguide mode.

A disadvantage of conventional waveguides used for optical sensing isthat they do not provide an optical mode centred on a sensing layer.This problem is overcome by the leaky mode waveguide.

A limitation of the leaky mode waveguide is that leaky modes aresensitive to changes of the dimensions of the layers comprising thewaveguide. The waveguide must therefore be made with tight fabricationtolerances.

It is an object of the present invention to provide a waveguidestructure which overcomes or mitigates the above disadvantage.

According to a third aspect of the invention there is provided awaveguide structure comprising a medium disposed as a sensing layer, asecond layer of material having a refractive index greater than that ofthe medium, and a substrate, wherein the structure defines a waveguidecapable of supporting an optical mode confined in the sensing layer, themedium is adapted for performing chemical or biological reactions withinthe medium which will result in a change of an optical property of thesensing layer of the waveguide, and the thickness and refractive indicesof the layers are chosen such that an optical mode confined in thesensing layer will suffer substantially anti-resonant reflection as aconsequence of the interface between the sensing layer and the secondlayer and the interface between the second layer and the substrate.

The sensing layer is bounded on one side by a material whose refractiveindex is lower than that of the sensing layer.

The reference in the statement of invention to a mode being confined inthe sensing layer of the waveguide structure is intended to mean thatthe mode is centred on that layer of the waveguide, and it will beappreciated that a proportion of the mode will extend beyond that layer.

The inventors have realised that anti-resonant reflecting opticalwaveguides (ARROW's) may be used to concentrate an optical field in asensing region having a low refractive index. Since biochemical sampleseparation, antibody-antigen interactions, etc. are usually carried outin low index layer (dextran gel, a polymer or other suitable medium),ARROW waveguides allow concentration of an optical field in a region inwhich a chemical or biological reaction is to take place (i.e. thesensing layer of the above waveguide structure).

According to a fourth aspect of the invention there is provided awaveguide structure comprising a medium disposed as a sensing layer, asecond layer of material having a refractive index greater than that ofthe medium, and a substrate, wherein the structure defines a waveguidecapable of supporting an optical mode confined in the sensing layer, themedium is adapted for performing chemical or biological reactions withinthe medium which will result in a change of an optical property of thesensing layer of the waveguide, and the thickness and refractive indicesof the layers are chosen such that an optical mode confined in thesensing layer will suffer substantially resonant reflection as aconsequence of the interface between the sensing layer and the secondlayer and the interface between the second layer and the substrate.

The use of a resonant reflection to confine the optical mode, ratherthan an anti-resonant reflection, is advantageous because it renders theoptical mode more sensitive to a change of an optical property of thesensing layer of the waveguide. Waveguides configured to provide anoptical mode confined by resonant reflection are hereafter referred toas resonant optical waveguides (ROW's).

The medium adapted for performing chemical or biological reactions inthe ARROW or ROW waveguides is preferably dextran gel, but may be anyother suitable low-index material.

Preferably, the ARROW or ROW waveguide structure is adapted for use aspart of an optical sensing apparatus.

Preferably, the optical sensing apparatus comprises the waveguidestructure, an optical source, means for coupling light from the opticalsource into an optical mode confined in the sensing layer of thestructure, and means for detecting changes in the properties of theoptical mode by monitoring properties of light coupled from thewaveguide structure.

Preferably, the coupling means comprises a prism which is locatedagainst or adjacent the substrate of the waveguide structure, the prismbeing configured to allow light to be coupled into a resonant opticalmode confined in the sensing layer of the structure, when the light isincident upon the prism at a predetermined angle. A change of therefractive index of the sensing layer of the structure will modify theangle which will excite a resonant mode of the waveguide structure

Preferably, the optical sensing apparatus is provided with means forscanning the light from the optical source so that it is incident at thewaveguide over a range of incident angles. This may be done for exampleby mounting the optical source on a swinging arm. In the alternative,means may be provided to direct light from the optical source onto thewaveguide from many angles simultaneously.

Preferably, the optical sensing apparatus is provided with means forproviding light capable of exciting both TE and TM modes confined in thesensing layer of the waveguide structure, and means for producinginterference between light coupled from the TE and TM modes, once it hasbeen coupled out of the waveguide structure.

The optical source used to excite an ARROW waveguide mode may be a lightemitting diode, or may be capable of producing a broad spectrum ofwavelengths of light. The use of a light emitting diode, or a whitelight source, is made possible by the relative insensitivity of theARROW mode index to variations of the wavelength of incident light. If anarrow wavelength band of incident light is required, a laser may beused as the light source. A narrow wavelength band will be required toexcite a ROW waveguide mode.

The optical apparatus may include means for detecting a dip in theintensity of the light coupled from the waveguide. Should the waveguidestructure cause scattering or absorption of light confined within thesensing layers a dip in the intensity of light coupled from thewaveguide will indicate the presence of a waveguide mode.

The waveguide structure may be provided with a low index spacer layerlocated between the second layer and the substrate. The low index spacerlayer is advantageous because it allows ARROW modes and resonant mirrormodes to be excited in a single waveguide, thereby allowing comparisonbetween them. The low index spacer may similarly allow the simultaneousexcitation of ROW modes and resonant mirror modes.

The waveguide structure may be arranged to cause scattering orabsorption by the introduction of scattering or absorbing elements inthe sensing layer or the second layer of the waveguide, or by providingeither of those layers with roughened surfaces. Where the waveguidestructure includes a low index spacer layer. scattering or absorbingelements may be introduced into the spacer layer. The spacer layer maybe provided with roughened surfaces.

The waveguide structure may be provided with a further layer spacedapart from the second layer by a layer of lower refractive index, thefurther layer having a refractive index greater than that of the sensinglayer. The introduction of this extra layer will decrease the lossessuffered by a mode confined in the first layer of the waveguide, andwill decrease the range of angles of incident light which may be used toexcite a resonant mode confined in the sensing layer of the waveguidestructure.

The waveguide structure may be provided with a fourth layer located onan uppermost surface of the sensing layer, the fourth layer beingmaterial with a similar refractive index to the second layer, and afifth layer of substrate located on top of the fourth layer. The sensinglayer will thus effectively be bounded on both sides by ARROW or ROWstructures. This structure may be referred to as a symmetric ARROWstructure or symmetric ROW structure, although the corresponding layerson either side of the sensing layer are not required to be of identicalthickness or to have the same refractive index. In this configuration,the sensing layer may consist of a fluid that may be allowed to flowthrough the waveguide structure. This configuration allows an opticalmode to be confined in the fluid, and thereby allows the properties ofthe fluid to be monitored.

Optical sensing apparatus for use with a waveguide comprising the abovesymmetric waveguide structure may include means for detecting a dip inthe intensity of the light coupled from the waveguide. Resonant modes ofthe waveguide will be manifest as dips in the intensity of lightreflected from the waveguide structure or peaks in the intensity oflight transmitted by the waveguide structure

The optical apparatus may be configured to detect the presence of gasesor chemicals suspended in the air, water or other fluid. One way inwhich this may be done is by forming the sensing layer of the waveguidestructure from a polymer or other material whose refractive index,density or other property is sensitive (i.e. altered) by the presence ofthat chemical or biochemical species that is to be detected.

The optical apparatus may be arranged to monitor changes of therefractive index of the sensing layer of the waveguide structure, oralternatively the apparatus may be arranged to monitor fluorescence orabsorption within the sensing layer.

According to a fifth aspect of the invention there is provided a methodof optical sensing, comprising coupling light into a mode confined inthe sensing layer of a waveguide structure described in accordance withthe third aspect of the invention or the fourth aspect of the invention,coupling light out of the waveguide structure using a prism, andmonitoring the angle at which coupling of light to the mode passesthrough a resonance.

The method may include coupling white light into a mode confined in thesensing layer of the waveguide structure described in accordance withthe third aspect of the invention, thereby allowing the spectroscopicanalysis of biological samples.

It is an object of the present invention to provide an alternativewaveguide structure which supports an optical mode centre on a sensinglayer.

According to a sixth aspect of the invention there is provided awaveguide comprising a sensing layer of a medium, a second layer forminga lower surface of the medium and having a refractive index greater thanthat of the medium, and a third layer forming an upper surface of themedium and having a refractive index greater than that of the medium,wherein the medium is adapted for performing chemical or biologicalreactions within the medium which will result in a change of an opticalproperty of the sensing layer of the waveguide, and the waveguide iscapable of supporting an optical mode centred on the sensing layer.

The waveguide, which will be referred to as a light condenser, isadvantageous because its structure is very simple, and it is robust withrespect to environmental changes (for example temperature fluctuations).

The light condenser mode is centred on the sensing layer, therebyproviding sensitive measurement of changes of the optical properties ofthe medium comprising the sensing layer.

According to a seventh aspect of the invention there is provided awaveguide comprising a sensing layer of a medium, a second layer forminga lower surface of the medium and having a refractive index greater thanthat of the medium, and a third layer forming an upper surface of themedium and having a refractive index less than that of the medium,wherein the medium is adapted for performing chemical or biologicalreactions within the medium which will result in a change of an opticalproperty of the sensing layer of the waveguide, and the waveguide iscapable of supporting an optical mode centred on the sensing layer

The waveguide according to the seventh aspect of the invention providesa light condenser reflection at the interface between the layer ofmedium and the second layer, and conventional total internal reflectionat the interface between the layer of medium and the third layer.

A known construction of optical sensor, referred to as a resonant mirrorbiosensor, attempts to combine the sensitivity of waveguiding deviceswith the simple construction and use of surface plasmon resonancedevices (see Cush, R. et al (1993) The resonant mirror, Biosensors &Bioelectronics, 8, 347-353). The resonant mirror biosensor is similar inconstruction to a surface plasmon resonance device. A sensing layer,i.e. the material whose optical properties are to be monitored, isplaced in contact with a high refractive index layer. The refractiveindex and thickness (typically about 100 nm) of the high index layer areselected in such a way that the sensitivity of the sensor is maximised.This high index layer is separated from a prism by a layer of lowerrefractive index material, called the spacer layer (e.g. silica). Therefractive index and thickness (typically about 0.5 microns) of thelower index layer are selected such that the sensitivity of the sensoris maximised and/or the sharpness of the Resonant Mirror resonances aremaximised. The sensitivity of the sensor and sharpness of the modes canbe controlled by altering the refractive index or thickness of the highindex layer and spacer layer. The refractive index of the prism alsocontrols the sensitivity of the sensor and sharpness of the modes. Therefractive index of the prism must be higher than the mode index of theResonant Mirror modes.

The resonant mirror differs from conventional waveguide sensors in thatthe mode excited in the waveguide sensor is leaky in nature. Thisfeature, which may also be seen in surface plasmon resonance waveguides,allows light to be coupled into and out of the resonant mirror via theprism.

Efficient coupling of light to the high index dielectric layer occursonly for certain angles of incident light where phase matching betweenan incident beam and resonant modes of the high index dielectric layeris achieved. At a resonant point, light couples into the high indexdielectric layer and propagates some distance along the sensinginterface before coupling back into the prism. An evanescent waveassociated with the resonant modes of the high index dielectric layerwill extend into the sensing layer. Changes of optical properties of thesensing layer will alter the properties of the resonant modes of thehigh index dielectric layer. Generally, the thickness of the high indexdielectric layer is made very low, in order to maximise the proportionof the optical mode in the evanescent field interacting with the sensinglayer, and so maximise the sensitivity of the device. The thinwaveguiding layer generally provides a single waveguide mode (one TEmode and/or one TM mode).

Leaky resonant mirror modes in the resonant mirror biosensor may existfor both TE and TM polarisations, and are seen as fine structure in thereflected light once it has passed through an output analyser. Theangles of incidence which excite modes of the high-index layer aresensitive to changes in the sensing layer, and so changes caused byassay reactions in the sensing layer may be monitored by measuringshifts in the excitation angle.

A limitation of resonant mirror waveguides is that a variation in thewavelength of light incident at a waveguide will alter the angle ofincidence required to excite resonant modes of that waveguide. Theeffect of a change of incident wavelength cannot be separated from theeffect of a refractive index change in the sensing region, and thesensitivity of an optical sensor comprising the resonant mirror is thuslimited by the extent to which variations of the wavelength of incidentlight can be suppressed. Lasers are used to provide the narrowwavelength band of night required for resonant mirror optical sensors.Unfortunately, lasers are susceptible to an effect known as ‘modehopping’ wherein the laser wavelength jumps between different valueswhich satisfy the resonance, criteria of the laser structure. Thewavelength produced by a laser will also vary with temperature due tovariation of the dimensions of that laser. Known resonant mirror opticalsensors attempts to minimise the wavelength variations in the output ofa laser by providing a wavelength stabilisation mechanism. However, thismechanism is both complex and expensive.

Optical sensors comprising other optical waveguides structures may alsobe susceptible to wavelength changes.

It is an object of the present invention to provide a waveguidestructure which overcomes or mitigates the above disadvantage.

According to an eighth aspect of the invention there is provided anoptical sensor comprising a waveguide defined by a plurality of layersincluding a sensing layer comprising a sensing medium adapted forperforming chemical or biological reactions which will result in achange of an optical property of the sensing layer, the layers beingcapable of supporting at least one optical mode, wherein at least afirst component of a supported mode extends into the sensing layer to asubstantial extent such that the first component is affected by changesin optical properties of the sensing layer, and at least a secondcomponent of a supported mode does not extend into the sensing layer toa substantial extent such that the second component is not substantiallyaffected by changes in optical properties of the sensing layer, thesensor further comprising means for detecting variations in signalsrepresentative of the first and second components, and means forcomparing the detected signals to identify variations whichsubstantially affect only the first component.

The optical sensor is advantageous because the second component will besubstantially unaffected by the optical properties of the layer ofsensing medium, and may be used as a reference component. The firstcomponent will be affected substantially by the optical properties ofthe sensing layer, and may be used to measure said optical properties.An unwanted experimental variation, for example a change of wavelengthof light coupled to the waveguide, since it will affect both componentsequally, may be removed from a measurement of the optical properties ofthe medium by comparison of the measurement and reference components.

Suitably, the layers are capable of supporting two modes a first ofwhich is the first component and a second of which is the secondcomponent.

Preferably, the two modes are centred on different layers of thewaveguide.

The two modes may be resonant mirror modes. Alternatively, the two modesmay be anti-resonant reflecting optical waveguide (ARROW) modes,resonant optical waveguide (ROW) modes or light condenser modes. Otherdifferent forms of modes may be supported.

The optical sensor is advantageous for measurements utilising modesother than resonant mirror modes for the same reasons given above inrelation to resonant mirror modes.

An optical sensor, according to the invention, which is designed tosupport resonant mirror modes may have a sensing layer of semi-infinitethickness, or may have a sensing layer of finite thickness. In contrastto this, an optical sensor which is designed to support ARROW modes orROW modes must have a sensing layer of finite thickness, and cannot havea semi-infinite sensing layer.

The layers may be capable of supporting a single mode, a first portionof the single mode defining the first component which extends into thesensing layer, and a second portion of the single mode defining thesecond component which does not extend into the sensing layer.

According to a ninth aspect of the invention there is provided a methodof optical sensing comprising exciting at least one optical mode in awaveguide structure defined by a plurality of layers including a sensinglayer comprising a sensing medium adapted for performing chemical orbiological reactions which will result in a change of an opticalproperty of the sensing layer, wherein at least a first component of asupported mode is excited so as to extend into the sensing layer to asubstantial extent such that the first component is affected by changesin optical properties of the sensing layer, and at least a secondcomponent of a supported mode is excited so as not to extend into thesensing layer to a substantial extent such that the second component isnot substantially affected by changes in optical properties of thesensing layer, the method further comprising detecting variations insignals representative of the first and second components, and comparingthe detected signals to identify variations which substantially affectonly the first component.

Preferably, two modes are be supported by the layers, a first of whichis the first component and a second of which is the second component.The two modes may be centred on different layers of the waveguide.

The two modes may be resonant mirror modes, or anti-resonant reflectingoptical waveguide modes

A single mode may be supported, a first portion of the single modedefining the first component which extends into the sensing layer, and asecond portion of the single mode defining the second component whichdoes not extend into the sensing layer.

DESCRIPTION OF THE DRAWINGS

Specific embodiments of different aspects of the invention will now bedescribed by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic view from one side of a waveguide comprising partof an optical sensor;

FIG. 2 is a schematic illustration of an optical sensor;

FIG. 3 is a graph of reflected intensity against incident angle whichhas been calculated for the waveguide illustrated in FIG. 1

FIG. 4 is a graph of reflected intensity against incident angle whichhas been calculated for an alternative waveguide;

FIG. 5 is two graphs of reflected intensity against incident angle whichhave been calculated for two alternative waveguides;

FIG. 6 is a graph of reflected intensity against incident angle whichhas been calculated for an alternative waveguide;

FIGS. 7 to 11 are graphs of field amplitude which have been calculatedfor the waveguide of FIG. 6;

FIG. 12 is a graph of reflected intensity against incident angle whichhas been calculated for an alternative waveguide;

FIGS. 13 to 17 are graphs of field amplitude which have been calculatedfor the waveguide of FIG. 6;

FIG. 18 is a schematic view from one side of a waveguide comprising partof an optical sensor;

FIG. 19 is a schematic perspective view of a waveguide structures,

FIG. 20 is a graph illustrating the confinement of an optical modewithin a waveguide structure corresponding to that illustrated in FIG.19;

FIG. 21 is a schematic view from one side of an optical sensingapparatus;

FIG. 22 is a refractive index profile of a waveguide structure;

FIG. 23 is a graph illustrating the confinement of an optical modewithin a waveguide structure corresponding to that illustrated in FIG.22;

FIG. 24 is a schematic view from one side of a waveguide structure;

FIG. 25 is a graph illustrating the confinement of an optical modewithin a waveguide structure corresponding to that illustrated in FIG.24.

FIG. 26 is a refractive index profile of an alternative waveguidestructure;

FIG. 27 is a refractive index profile of an alternative waveguidestructure;

FIG. 28 is a schematic illustration of a light condenser waveguidestructure;

FIG. 29 is a schematic illustration of an alternative light condenserwaveguide structure;

FIG. 30 is a schematic view from one side of a waveguide structure;

FIG. 31 is a schematic illustration of an optical sensor incorporatingthe waveguide of FIG. 30;

FIG. 32 is a diagram showing a first mode supported by the waveguidestructure of FIG. 30;

FIG. 33 is a diagram showing a second mode supported by the waveguidestructure of FIG. 30;

FIG. 34 is graph representing a series of outputs from the opticalsensor shown in FIG. 31;

FIG. 35 is graph representing a further series of outputs from theoptical sensor shown in FIG. 31;

FIG. 36 is a schematic view from one side of a waveguide structurecomprising part of an optical sensor;

FIG. 37 is a diagram showing a first mode supported by the waveguidestructure of FIG. 36;

FIG. 38 is a diagram showing a second mode supported by the waveguidestructure of FIG. 36;

FIG. 39 is a schematic view from one side of a waveguide structurecomprising part of an optical sensor;

FIG. 40 is a diagram showing a first mode supported by the waveguidestructure of FIG. 39; and

FIG. 41 is a diagram showing a second mode supported by the waveguidestructure of FIG. 39.

DESCRIPTION OF PREFERRED EMBODIMENTS

A waveguide 1 comprising part of the optical sensor according to thefirst aspect of the invention is shown in FIG. 1. The waveguide 1comprises a dielectric prism substrate 2, a Zirconium layer 3 depositedon an upper surface of the prism substrate 2 and a sensing layer ofdextran gel 4. The Zirconium layer 3 is 20 nm thick, and the layer ofdextran gel 4 in the illustrated waveguide is 800 nm thick and has arefractive index of 1.39. The prism substrate is effectivelysemi-infinite in thickness and has a refractive index of 1.72. Anuppermost surface of the layer of dextran gel 4 is in contact with water5 (refractive index 1.33) which is effectively semi-infinite inthickness.

The waveguide 1 is similar to waveguides which are used for surfaceplasmon resonance measurements. However, whereas a medium of interestused for surface plasmon resonance measurement may be semi-infinite(e.g. water), the optical sensor according to the invention requiresthat the medium of interest be disposed as a layer, as for example thelayer of dextran gel 4 in FIG. 1.

When light is incident on the waveguide 1 at a specific angle a leakymode will be excited. The mode is excited by light at a wavelength of619.9 nm, which is incident at a particular angle on the prism substrate2 of the waveguide, such that it couples through the Zirconium layer 3and into a dextran gel layer 4. The mode is centred on the dextran gellayer 4, although a significant proportion of the amplitude extendsbeyond that layer and into the semi-infinite layer of water 5. The modemay be described as leaky, in the sense that a proportion of the lightpropagating in the mode will couple back into the prism 2. It is thisleaky property which allows excitation of the mode through the prismsubstrate 2. The mode will hereafter be referred to as a leaky waveguidemode.

An optical sensor according to the invention is shown in FIG. 2. Thesensor is similar to existing apparatus which is used to perform surfaceplasmon resonance measurements. The apparatus according to the inventioncomprises a light source 6, a lens 7 which directs a fan-shaped beam oflight through a prism substrate 2 which forms part of a waveguide 1 (thewaveguide corresponds to the waveguide shown in FIG. 1). Because thelight is directed towards the waveguide 1 as a fan, light is incident atthe waveguide 1 from a range of different angles. The prism substrate 2is chosen to have a refractive index such that modes of the waveguide 1are leaky, that is the modes couple into and out of the waveguide 1easily. Although the prism substrate 2 is shown as being triangular, itcould be of any suitable shape (for example rectangular), and otherforms of substrate may be used. The incident light will either becoupled to a leaky waveguide mode centred on a layer of dextran gel 4,or will be reflected from the waveguide 1 without being coupled to theleaky waveguide mode, and then coupled out of the waveguide 1. Lightwill be coupled from the prism 2 of the waveguide 1 in the form of afan, and will be incident upon a detector 8. The detector 8 comprises anarray of charge coupled devices (CCD's) which detect the intensity oflight at different sections of the fan.

The waveguide 1 is dimensioned such that the field amplitude of light inthe Zirconium layer 3 is less for light incident at an angle whichresults in excitation of a leaky waveguide mode than for light incidentat an angle which does not result in excitation of a leaky waveguidemode, and the intensity of light incident at the detector 8 thereforepeaks upon excitation of a leaky waveguide mode.

If a leaky waveguide mode is excited in the waveguide 1 for a particularangle of incident light, this will be seen as a peak in the intensity oflight incident at the detector 8 at one position. The position of thepeak in intensity is dependent upon the refractive index of the prism 2and on the optical properties of the layer of dextran gel 4. A chemicalor biochemical reaction which modifies the optical properties of thedextran gel 4 may be monitored in real time by detecting movement of thepeak.

The CCD array of the optical sensor may be replaced by a singlephoto-diode (not shown) mounted so as to be capable of translation in adirection perpendicular to the direction of the light reflected from thewaveguide 1. In use the photo-diode would be positioned at the locationof a peak in intensity, and would be translated to follow the peak ofintensity during an experiment, thereby allowing measurement of thedegree of movement of that peak in intensity.

It will be appreciated that the combination of the tight source 6 andlens 7 of FIG. 2 may be replaced by a light source of much smaller area,mounted on a swinging arm. The arm would be swung through a requiredrange of angles to produce illumination at the waveguide similar to thefan of light shown in FIG. 2. The lens 7 would not be required by theswinging arm arrangement.

FIG. 3 shows a graph of reflected intensity against incident angle,which has been calculated for the waveguide illustrated in FIG. 1 toillustrate the operation of the optical sensor according to theinvention. A sharp peak in intensity is seen at approximately 53degrees, which corresponds to a leaky TE waveguide mode.

Other examples of leaky waveguide structures which provide a peak ofintensity for 619.9 nm light at resonance comprise a semi-infinitesubstrate (refractive index 1.72), one of the following metals or metalalloys:

Metal/Metal Alloy Thickness Chromium 3 nm Manganese 5 nm Molybdenum 4 nmNickel 5 nm Niobium 5 nm Platinum 4 nm Ruthenium 3 nm Tantalum 9 nmTellurium 3 nm Titanium 7 nm Tungsten 5 nm Vanadium 7 nm Zirconium 20nm 

an 800 nm thick layer of dextran gel (refractive index 1.39), and asemi-infinite layer of water (refractive index 1.33).

FIG. 4 shows a graph of reflected intensity against incident angle,which has been calculated for a leaky waveguide structure having a 5 nmthick layer of Tungsten (with other layers and dimensions as describedabove). A sharp peak in intensity is seen at approximately 53 degrees,which corresponds to a leaky TE waveguide mode.

FIG. 5 shows a graph of reflected intensity against incident angle,which has been calculated for a leaky waveguide structure having a 3 nmthick layer of Chromium (with other layers and dimensions as describedabove for FIG. 5a, and with a 2000 nm thick layer of dextran gel for inFIG. 5b). A sharp peak in intensity is seen in FIG. 5a at approximately53 degrees, which corresponds to a leaky TE waveguide mode. Three sharppeaks in intensity are seen in FIG. 5b between 50 and 54 degrees, whichcorrespond to three leaky TE waveguide modes.

The invention is advantageous because the precise detection of thelocation of a peak of intensity is more easily achieved than thedetection of the location of a dip in intensity (prior art leaky modewaveguides provide only dips in intensity).

The inventors have realised that the generation of a peak of outputrather than a dip of output from a leaky mode waveguide is determined bythe field amplitude of light in the metal layer of the waveguide. Lighthaving a large field amplitude in the metal layer of the leaky modewaveguide will suffer significant loss, as energy is deposited as heatin the metal. A leaky mode waveguide may be configured to provide a peakof output by arranging the waveguide such that the amplitude of the modein the metal layer is low when the leaky mode is excited.

FIGS. 6 to 17 show results from a computer model indicating how a leakymode waveguide may be configured to provide a peak of output. Aconventional leaky mode waveguide having the following dimensions:

Region Refractive index Thickness Label Substrate 1.72 Semi-infinite 9Metal (Gold) 0.13-3.16i 0.05 microns 10 Sensing layer 1.38  2.0 microns11 Superstrate 1.333 Semi-infinite 12

is excited with incident light at 619.9 nm. The angle of incidence ofthe light is measured relative to a normal from the plane of the metallayer 10.

A plot of reflectivity against angle of incidence for the waveguidedescribed in the table is shown in FIG. 6. The dips in the plot are dueto leaky modes being excited in the sensing layer 11. In this case, (asin prior art leaky mode waveguides) when the modes are excited, theoverlap between the mode and the metal layer 10 increases. This leads tooptical power loss, and this manifests itself as dips in thereflectivity.

FIGS. 7 to 11 illustrate the optical field amplitude in the waveguidedescribed in the table, for a variety of angles of incident light. FIG.7 shows the amplitude of the optical field in the waveguide when theangle of incidence is 51.5 degrees. The amplitude is normalised, withthe average amplitude of incident light being set at 1. Light in thesubstrate 9 of the waveguide is simply laser light at 619.9 nm andoscillates between 0 and 2. The normalised amplitude of light in thegold layer 10 is approximately 0.3. The normalised amplitude of light inthe sensing layer 11 is approximately 0.3, and decays gradually into thesubstrate 12. A leaky mode of the waveguide is not excited by the lightincident at 51.5, as is indicated by the low amplitude of light in thesensing layer 11.

FIG. 8 shows the amplitude of the optical field at an angle of resonanceof 51.94485 degrees (i.e. an angle of incident light at which a leakymode is excited). The leaky mode is clearly shown by the fact that theoptical field in the sensing layer 11 is approximately 35. The excitedmode is a second order leaky mode of the waveguide. The averageamplitude of the field in the metal at this angle of incidence isapproximately 2. This means that more energy is being deposited as heatin the metal layer 10 at this angle of incidence than at 51.5 degrees,and the reflectivity of the waveguide is correspondingly reduced. Thisexplains the presence of the dip of reflectivity seen at 51.94485degrees in FIG. 6.

FIGS. 9 and 10 show the amplitude of the optical field in the waveguidefor incident light at 52.5 degrees (no leaky mode excitation) and53.99036 degrees (first order leaky mode excited) respectively. Theaverage amplitude of the field in the metal layer 10 in FIG. 9 isapproximately 0.3. This is a relatively low value, and so thereflectivity at this point is relatively high. In FIG. 10, the fieldamplitude in the metal layer 10 is approximately 1.5, so that there is adip in reflectively at this point. FIG. 11 shows the amplitude of theoptical field in the waveguide for incident light at 53.5 degrees. Theamplitude of the field in the metal layer 10 is low (approximately 0.3),so that the reflectivity of the waveguide is relatively high.

As a further example of the present invention, a leaky mode waveguidewas prepared having the following dimensions:

Region Refractive index Thickness Label Substrate 1.72 Semi-infinite 13Metal (Chromium) 0.48-4.36i 0.005 microns 14 Sensing layer 1.38  2.0microns 15 Superstrate 1.333 Semi-infinite 16

and excited with incident light at 619.9 nm. Again, the angle ofincidence of the light is measured relative to a normal from the planeof the metal layer 14.

A plot of reflectivity against angle of incidence for the waveguidedescribed in the above table is shown in FIG. 12. In this case, thereare peaks rather than dips in the reflectivity plot, which correspond tothe presence in the waveguide of leaky modes. The reason for this can beseen from the FIGS. 13 to 17, which illustrate the amplitude of theoptical field as a function of angle of incidence.

FIGS. 14 & 16 show leaky modes excited in the waveguide at 51.92151degrees (a second order leaky mode) and 52.9841 degrees (a first orderleaky mode). At these angles, the amplitude of the optical field in themetal layer 14 is almost zero. Therefore a negligible amount of energyis removed from the system and so the reflectivity of the waveguide isclose to 1.0 when the leaky modes are excited. For light incident at51.0, 52.5 and 53.5 degrees (FIGS. 13, 15 & 17) the amplitude of theoptical field in the metal layer 14 is relatively high, and so thereflectivity at these points is relatively low.

The use of a leaky mode waveguide is advantageous in that a variation ofthe wavelength of incident light will not produce a significantvariation in the angle of incidence required to excite a leaky waveguidemode. This contrasts with surface plasmon resonance, wherein a variationof the wavelength of incident light will introduce an error into anangular measurement. Sources of incident light other than lasers may beused without significant loss of resolution, for example a lightemitting diode or other broad band source. The use of a light emittingdiode, which produces light over a wider band of frequencies than wouldbe produced by an equivalent laser, is made possible by the relativewavelength insensitivity of the leaky waveguide mode. Light emittingdiodes and other broad band sources are advantageous because they do notsuffer from ‘speckling’, which degrades the performance of instrumentsthat use diode lasers. The use of a broad band light source will allowthe spectroscopic analysis of biological samples—something that isdifficult or impossible to do with surface plasmon resonance sensors.

Since leaky waveguide modes of TE or TM polarisation may be excitedaccording to the invention, control of the polarisation of incidentlight is not necessary. Polarisation control is preferred because itallows the relative magnitudes of the TE and TM modes to be fixed. Thiscontrasts with surface plasmon resonance waveguides, wherein only one TMmode may be excited and the incident light is polarised accordingly.

An alternative form of leaky mode waveguide is illustrated in FIG. 18.The waveguide comprises a layer of dextran gel 17 bounded on either sideby a metal layer 18 and a substrate 19. A mode (or modes) may be excitedin the waveguide shown in FIG. 18 in the manner described above. Themode will be more tightly confined within the layer of dextran gel 17than a mode confined in the waveguide structure shown in FIG. 1.

Waveguides of the form shown in FIG. 18 may be described as symmetric,although it is not necessary to their operation that they be strictlysymmetric. A feature of symmetric waveguides is that, as well asreflection of incident light, they also provide transmission of incidentlight, since modes of the waveguides may be arranged to be leaky on bothsides of the gel layer 17. This is not the case with waveguides of theform shown in FIG. 1, in which transmission of light is inhibited.

A construction of symmetric waveguide with a central layer consisting offluid (rather than the dextran gel 17) may be used to detect refractiveindex changes in that fluid. Changes of the refractive index of thefluid are monitored using the techniques described above.

The leaky mode waveguide may be used in the measurement of fluorescence(by including a fluorescent species in the sensing layer).

An alternative waveguide structure which may be used as an opticalsensor is shown in FIG. 19. The waveguide is an Anti-Resonant ReflectingOptical Waveguide (ARROW). ARROW's are a class of waveguide whichexhibit special propagation characteristics that make them suited tooptical sensing applications.

ARROW waveguides were first developed in 1986 at AT&T Bell labs, and aredescribed in the paper: Duguay et al, Appl. Phys. Lett., 49 (1986)13-15. The waveguide 20 shown in FIG. 19 is an ARROW structure, andcomprises a sensing layer 21 (approx. 4 μm) of low refractive index gel(or other low index substance of interest) situated on top of a thinhigh index layer 22 (approx. 0.1 μm), which in turn is located on top ofa layer of silica 23 (approx. 0.5 μm). The entire structure is supportedon a transmissive substrate 24 (for example glass). Light propagating inthe sensing layer 21 of the waveguide 20 will undergo total internalreflection at an interface between an upper surface of the sensing layer21 and the surrounding air or other low index medium, and undergo veryhigh reflection from high index layer 22. The high index layer 22 actsas a Fabry-Perot resonator at anti-resonant wavelengths, providing avery high degree of confinement of the optical mode within the sensinglayer 21.

It is within the sensing layer 21 of the waveguide 20 that molecularinteractions (or any other interactions) which are to be studied occur.The waveguide 20 thus exploits an important advantage of ARROWstructures, namely that they allow concentration of an optical field ina low refractive index region of interest. This feature is importantsince biological sample separation, antibody-antigen interactions etc.are usually carried out in low index dextran gel. The waveguide 20allows an optical field to be concentrated in the dextran gel (i.e.sensing layer 21), whereas in most known prior art waveguides the fieldis localised in a high refractive index layer adjacent to the dextrangel. The enhanced overlap in the waveguide 20 between the optical fieldand the region to be monitored provides significantly increasedsensitivity.

The waveguide 20 is easy to fabricate, and dispersion characteristics ofARROW modes are such that even a quite large variation in waveguideparameters, i.e. layer thickness or refractive index, does notsignificantly affect the operation of the waveguide. This is asignificant advantage of the invention, since conventional knownwaveguides are very sensitive to variation of waveguide parameters.

FIG. 20 shows an optical sensing apparatus which utilises the waveguide20 illustrated in FIG. 19. The construction of the apparatus is basedupon the construction of the known resonant mirror biosensor (see Cush,R. et al (1993) The resonant mirror, Biosensors & Bioelectronics, 8,347-353). The apparatus comprises a source 25 which produces a beam oflight at a known wavelength. The beam is collimated, and then polarisedby a polariser 26 to provide equal proportions of TE and TM excitationbefore being focused into a prism 27. The beam is coupled from the prism27 into the waveguide 20 via leakage of the modes of the waveguide 20.Efficient coupling into the sensing layer 21 of the waveguide 20 willoccur only for certain angles of incidence where phase matching betweenthe incident beam and resonant modes of the sensing layer 21 isachieved. The angle at which the beam is incident upon the prism 19 isscanned continuously through a predetermined range, which is chosen toinclude those angles needed to excite resonant modes of the core layer.An alternative arrangement of apparatus couples incident light to thewaveguide 19 in a wedge shape, thereby providing light simultaneously ata range of incident angles which include all angles of interest.

The incident angle which will provide efficient coupling to the sensinglayer 21 of the waveguide 20 (i.e. the angle which will excite a mode ofthe waveguide 20) is dependent upon the refractive index of the sensinglayer 21 and on the refractive index of the substrate 24 and prism 27.Molecular interactions occurring within the sensing layer 21 will modifythe refractive index of the core 21 and thereby change the incidentangle required for efficient coupling. This change in refractive indexmay be monitored by measuring changes in the angle which providesefficient coupling.

Light which is coupled into the sensing layer 21 propagates a shortdistance along it before coupling back into the prism 27. Light emittedfrom the prism 27 may be collimated and then caused to pass through ananalyser 28 comprising a polariser set at 45° to the axes ofpolarisation of the TE and TM components of the light and a quarter-waveplate. A detector 29 measures the position of fringes produced byinterference between the TE and TM components of the light passed by theanalyser 28.

The phase of light reflected by the waveguide 20 undergoes a full 2πchange on passing through a resonance peak (i.e. an angle of incidencewhich provides efficient coupling to the sensing layer 21). It is theposition of this phase step which is monitored to measure changes in theoptical properties of the sensing layer 21. The resonant optical modesfor TE and TM excitation are widely separated. As the angle of theincident light approaches the angle needed to excite, for example, aresonant TE mode, the phase of light coupled from the core layer will beshifted, and will pass through a maximum phase shift of π at theresonance peak. Light which is coupled to a TM mode of the sensing layer21 at the same angle of incidence will not pass through a resonant mode,and interference at the analyser between light coupled from the TE andTM modes of the sensing layer 21 will be modified by the π phase shiftof the TE mode, thereby indicating the presence of the TE resonance.

The ARROW waveguide is advantageous over known optical sensing apparatusin that the resonant modes of the waveguide 20 are almost wavelengthinsensitive, thereby removing the need for coherent sources of light tobe provided with wavelength stabilisation mechanisms,

The source 25 used by the ARROW waveguide may be either a laser a lightemitting diode or a white light source. The use of a light emittingdiode (or a white light source), which produces light over a wider bandof frequencies than would be produced by an equivalent laser, is madepossible by the wavelength insensitivity of the ARROW structure of thewaveguide 20. Light emitting diodes and other broad band sources areadvantageous because they do not suffer from ‘speckling’, which degradesthe performance of instruments that use diode lasers. The use of a whitelight source will allow the spectroscopic analysis of biologicalsamples—something that is difficult or impossible to do withconventional waveguide sensors.

The ARROW waveguide is advantageous in that the method of excitation ofmodes of the ARROW waveguide and the method of detection is the same asis currently used for RM modes in the known resonant mirror biosensor(see Cush, R. et al). ARROW waveguides may therefore be used in place ofresonant mirror waveguides in existing apparatus to obtain enhancedmeasurement sensitivity, without requiring a substantial change ofinstrumentation.

FIG. 21 shows the real part of the amplitude of an optical field in thewaveguide of FIG. 19. For comparison, two modes of the waveguide areshown: a resonant mode which occurs when the waveguide is acting as aresonant mirror, and a mode which occurs when the waveguide is acting asan ARROW waveguide. From FIG. 21 it can be seen that the overlap betweenthe ARROW mode and the sensing layer 21 of gel is almost 100% whilst theoverlap of the resonant mirror (RM) mode and the sensing layer 21 isabout 40%. Any change in the refractive index of the gel of sensinglayer 21 therefore has a greater effect on the ARROW mode than it doeson the RM mode. The ARROW mode thus provides more sensitive detectionthan the RM mode, thereby providing the sensing apparatus with anenhanced performance when compared to RM sensors.

A further advantageous feature of the ARROW waveguide is that theleakage rate associated with the ARROW mode is much lower than thatassociated with the RM mode. This means that the ARROW mode resonancesare much sharper than RM resonances (although the leakage rate of RMmodes can be reduced by increasing the thickness of the silica layer23).

The shift in the resonance angle of an ARROW resonance in response to achange in the refractive index of the core 21 was found to be 1.8 timesgreater than that of the corresponding RM resonance. With a tuned ARROWstructure however, this figure may be increased to over 12. Thisenhanced shift of resonance angle, together with the relative sharpnessof the ARROW modes, allows the sensing apparatus comprising an ARROWwaveguide to resolve much smaller changes in the refractive index of acore layer than may be measured using conventional RM sensors.

A large overlap between the optical field and a core layer is apre-requisite for efficiefit fluorescence and absorption measurements.The ARROW waveguide provides this large overlap. Since the overlapbetween the ARROW mode and the sensing layer 21 is almost 100%, theARROW waveguide is particularly suited to fluorescence and absorptionmeasurements.

The silica layer 23 in the waveguide of FIG. 19 is included so that bothresonant mirror modes and ARROW modes of the waveguide 20 may beexcited, thereby allowing comparisons of their properties. However, itis possible to fabricate an ARROW waveguide which does not include thesilica layer 23. A waveguide of this form will be unable to supportresonant mirror modes, but will support ARROW modes in the mannerdescribed above.

Although in general it is advantageous to produce waveguides whichsuffer as little absorption loss as possible, losses suffered by lightin an ARROW waveguide mode may be of some use. Specifically, when a modeof an ARROW waveguide is excited, absorption suffered by the light inthe waveguide mode will reduce the intensity of light coupled from thewaveguide, when compared to light which is not coupled to a resonantmode of the waveguide. Thus, the presence of an ARROW mode will beindicated by a dip in the intensity of light coupled from a waveguide.

Since a mode may be detected as a change of intensity rather than as achange of phase, the angle of resonance may be determined without usingthe polariser 26 or analyser 28. The size of the dip in the intensity oflight coupled from the waveguide is a function of the losses suffered bythe mode in the waveguide, either by absorption or scattering. Adisadvantage of absorption losses is that they will broaden resonancesof an ARROW waveguide, thereby reducing measurement sensitivity.

Optical absorption losses may be induced by introducing absorbing dyeswithin the core and/or high index layers. Equivalent losses may beinduced by providing a degree of roughness to one or more of thesurfaces of the core or high index layers.

For a given set of waveguide parameters there is an optimal value ofthickness of the high index reflector (layer 22 in FIG. 19), at whichthe leakage rate for a particular ARROW mode is a minimum. Minimisingthe leakage rate will reduce the width of the ARROW resonance to aminimum (the ARROW resonance is the range of angles of incidence whichexcite the ARROW mode). In many cases, it is advantageous to minimisethe width of the ARROW resonance, since this will maximise measurementsensitivity.

A simple ARROW structure with a refractive index profile is shown inFIG. 22. The reference numerals of FIG. 22 correspond with the referencenumerals applied to the structure shown in FIG. 19. For a structure ofthis type, the value of the optimum thickness, t, of the high-indexreflector layer is given to a good approximation by:$t = {\frac{\left( {{2N} + 1} \right)\lambda}{4n_{2}}\left\lbrack {1 - \left( \frac{n_{1}}{n_{2}} \right)^{2} + \frac{\lambda^{2}}{4n_{2}^{2}d^{2}}} \right\rbrack}^{- \frac{1}{2}}$$\begin{matrix}{N\text{:}} & \text{zero or a positive integer} \\{\lambda \text{:}} & \text{free-space wavelength} \\{{n1}\text{:}} & \text{refractive index of the core (guiding layer)} \\{{n2}\text{:}} & \text{refractive index of the high-index reflector layer} \\{d\text{:}} & \text{thickness of the core (guiding layer)}\end{matrix}$

For the simple ARROW waveguide shown, this formula gives a goodapproximation to the optimum value of t, i.e. the value that results ina minimum leakage rate. A fundamental mode of a simple ARROW structurewith the refractive index profile shown in FIG. 22 is illustrated inFIG. 23.

However, because the structure of ARROW sensors will not generally be assimple as that shown in this example, the value of t obtained by thisformula can be taken as a rough guide only. To determine the optimumvalue of the reflector thickness for any general ARROW structure, theARROW mode index may be numerically determined, e.g. by the transfermatrix method. The set of waveguide parameters that give the lowestleakage rate can then be determined.

In an alternative configuration of waveguide, the thicknesses andrefractive indices of the high index layer 22 and the substrate 24 isselected to act as a Fabry-Perot resonator at resonant wavelengths (i.e.the thickness of the high index layer 22 is a multiple of the wavelengthof light coupled to the waveguide, such that a maximum or near-maximumleakage of the optical mode occurs). This is in contrast to the designof ARROW waveguides, in which the structure is chosen such as tominimise the leakage of the optical mode. The waveguide configuration,referred to hereafter as a ROW waveguide provides strong confinement oflight in the sensing layer 21.

A feature of suitably tailored ROW waveguides is that the mode index isa strong function of the refractive index of the sensing layer 21. Themode index of the sensing layer 21 is generally referred to as A, andthe refractive index of the sensing layer 21 is generally referred to as‘n’. In ARROW waveguides, the quantity dβ/dn is approximately 1.0.However, in properly tailored ROW waveguides, dβ/dn may be significantlylarger than 1.0. In other words, a change of the refractive index of thesensing layer 21, for example as the result of a molecular interaction,will in general lead to a larger change in the optical properties of theguided mode than would be seen in an ARROW waveguide.

A drawback or ROW waveguides is that the mode index of ROW modes is astrong function of not only the refractive index of the sensing layer21, but also of the refractive index and thickness of all other regionsof the waveguide structure. Fabrication tolerance is therefore muchstricter for ROW waveguides than it is for ARROW waveguides. Anotherdrawback of ROW modes is that the mode index is a function of opticalwavelength, and monochromatic optical excitation of ROW modes is thisusually necessary. The main advantage of ROW waveguides is that theenhancement in sensitivity to changes of optical properties of a sensinglayer 21 can be very large. In the ROW structure used as an examplebelow, the value of dβ/dn is modest (approximately 1.09, compared withapproximately 1.0 for ARROW waveguides). With suitable design, thisvalue can be much higher

A ROW waveguide structure is illustrated in FIG. 24. The structurecomprises the following layers:

Layer Reference No. Region Refractive index Thickness (microns) 30Superstrate 1.00 31 Sensing layer 1.347 or 1.3471 4.0 32 Silicon nitride2.00 0.17 33 Silica 1.47 1.0 34 Metal 0.13-i3.16 0.015 35 Substrate  1.72038

The waveguide structure shown in FIG. 24 has been computer modelled andfound to operate as a ROW waveguide for light of 660 nm. When therefractive index of the sensing layer 31 is 1.3470, the real part of themode index of the TE₂ is 1.344562. When the refractive index of thesensing layer 31 is 1.3471, the real part of the mode index of the TE₂is 1.344671. This gives a value of dβ/dn=1.09.

The metal layer 34 is included in the waveguide structure merely toprovide optical loss and thereby allow the modes to be detected as dipsin the intensity of reflected light. The ROW waveguide structure ingeneral does not require a layer of metal.

A mode profile of the TE₂ ROW mode is shown in FIG. 25.

The preceding discussion and the following discussion of ARROW waveguidestructures may be applied, with relevant change, to ROW waveguidestructures.

A broad resonance may be obtained from an ARROW structure by detuningthe waveguide structure from the optimal ARROW configuration. This canbe done by adjustment of any one or any combination of the followingparameters: thickness of the high index layer, refractive index of thehigh index layer, refractive index of the substrate, refractive index ofthe sensing layer, thickness of the sensing layer, wavelength of theincident light.

If, on the other hand, it is required that the ARROW resonance besharper than that obtained using the optimal structure, further highindex reflector layers may be included in a waveguide. Any number ofhigh index reflector layers may be included in a waveguide structure.The refractive index profile of an ARROW waveguide which includes twohigh refractive index layers is shown in FIG. 26. The waveguidecomprises a substrate 36 on top of which is provided two high refractiveindex layers 37, 38 separated by a spacer layer 39. A sensing layer ofgel 40 is located on top of the uppermost high index layer 38. An uppersurface of the sensing layer 40 forms an interface with a superstrate 41of, for example, air. The refractive indices and thicknesses of thevarious layers of the waveguide shown in FIG. 26 may be varied in orderto achieve a desired sharpness of ARROW resonance. The thickness andlorthe refractive index of the high index layers 37 and 38 need not be thesame. The refractive index of the sensing layer 40 must be greater thanthat of the superstrate 41, but less than that of the high index layer38.

A further alternative form of ARROW waveguide structure is illustratedin FIG. 27. In this structure, a sensing layer 42 is bounded on eitherside by a high index layer 43, and a substrate 44. The structureillustrated in FIG. 27 will be referred to as symmetric, todifferentiate it from those structures described above, which aregrouped together under the description ‘asymmetric’. In asymmetric ARROWwaveguides, an uppermost surface of a sensing layer is bounded by asemi-infinite medium of lower refractive index (for example, air orwater), which boundary provides total internal reflection. By contrast,a sensing layer 42 of a symmetric ARROW waveguide is bounded on twosides by a high refractive index layer 43 and a substrate 44, which formtwo ARROW structures, oriented such that modes in the sensing layer 42are confined on both sides by ARROW confinement. The word ‘symmetric’ isintended to mean that the sensing layer 42 is provided on both sideswith an ARROW structure, and does not require that the refractiveindices of equivalent layers on either side of the sensing layer 42 areidentical, or that the form of the ARROW structure on either side of thesensing layer 42 be the same.

A feature of symmetric ARROW sensors is that, as well as reflection ofincident light, they also provide transmission of incident light. Thisis not the case with asymmetric ARROW sensors, in which transmission oflight is inhibited by total internal reflection at an uppermost surfaceof a sensing layer. In contrast to this, ARROW modes in symmetric ARROWwaveguides are leaky on both sides of the sensing layer 42. However, ifdesired, even a symmetric ARROW structure can be designed such that theARROW modes are leaky on only one side. This may be achieved by reducingthe refractive index of a superstrate of the structure to below the modeindex of the ARROW modes of interest.

Transmission of light by a symmetric ARROW waveguide will occur onlywhen light is incident on the waveguide at a resonant angle. The factthat a symmetric ARROW waveguide will transmit as well as reflect lightgreatly simplifies the measurement of the angle of incident lightrequired to excite a resonance of the ARROW waveguide. An apparatussimilar to that shown in FIG. 20 may be used with unpolarised light,thereby removing the need for polarisers at the input and output sidesof the waveguide. At ARROW resonance angles, there is a dip in thereflectivity and a peak in the transmissivity of the waveguide, eitherof which may be detected easily. The position of the transmissivity peakis measured using a prism on the transmission side of the waveguide, anda detector.

A construction of symmetric ARROW waveguide with a sensing layerconsisting of fluid may be used to detect refractive index changes inthat fluid. Changes of the refractive index of the fluid are monitoredusing the techniques described above.

Alternative techniques for coupling light into an ARROW waveguideaccording to the invention include end-fire coupling and coupling via agrating etched on an interface within the waveguide.

Although the invention has been described by way of example in terms ofsimple ARROW waveguides, it will be clear to those skilled in the artthat alternative more complicated configurations of ARROW waveguide maybe fabricated which also allow an optical mode to be confined in, forexample dextran gel or a polymer. Examples of such waveguides which maybe used as part of an optical sensing apparatus areDirectional-Couplers, Mach-Zehnder and other interferometric devices.

FIG. 28 shows a waveguide according to the sixth aspect of theinvention, which waveguide is referred to hereafter as a light condenserThe light condenser comprises a high index superstrate 45, a low indexsensing layer 46 and a high-index substrate 47. One possibleconfiguration of a light condenser comprises a low index gel or othermedium of interest with a refractive index of n=1.333, sandwichedbetween two layers of glass. The light condenser is a simple low-indexwaveguide with properties similar to ARROW waveguides.

Light is confined in the low-index guiding region of a light condenserby reflection from the index steps between the low-index sensing layer46 and the high-index superstrate 45 and substrate 47. Because therefractive index of the superstrate 45 and substrate 47 is higher thanthat of the sensing layer 46, modes of the light condenser are leaky innature. In other words the reflection from the core-cladding boundary isless than 100%, as indicated by light shown as arrows escaping throughthe superstrate 45 and substrate 47.

The guiding layer may be a polymer, water, gel or any other low-indexmaterial whose refractive index is to be monitored or in whichfluorescence is to be excited.

The waveguide will function as a light condenser provided that the realpart of the mode index of the light condenser mode is less than therefractive index of the superstrate 45 and substrate 47.

FIG. 29 shows a waveguide in which the refractive index of a substrate48 is greater than the refractive index of a sensing layer 49, and therefractive index of the superstrate 50 is less than the refractive indexof the sensing layer 49. Light is confined at the interface between thesensing layer 49 and the substrate by an index step, as described inrelation to FIG. 28. Light is confined at the interface between thesensing layer 49 and the superstrate 48 by conventional total internalreflection. A waveguide of this type will be referred to as anasymmetric light condenser. In the asymmetric light condenser the realpart of the mode index of a light condenser mode of interest isgenerally greater than the refractive index of the superstrate 48.

The waveguide will function as a light condenser at the interfacebetween the sensing layer 49 and the substrate 48, provided that thereal part of the mode index of the light condenser mode is less than therefractive index of the substrate 48.

The method of excitation of light condenser modes is the same as thatdescribed in relation to ARROW modes, and may utilise for example prismcoupling, grating coupling or end-fire coupling.

The light condenser does not generally require lateral mode confinement,since light coupled to the light condenser is collimated. Where lateralconfinement is required, this may be provided by etching into asubstrate or superstrate a channel for receiving a sensing medium.

The light condenser waveguide may be fabricated from injection mouldedplastic. A coupling prism may be formed together with the lightcondenser during fabrication. Electrodes, if required, may also beformed together with the light condenser during fabrication.

The light condenser waveguide may be used in the measurement offluorescence (by including a fluorescent species in the sensing layer).

FIG. 30 shows a resonant mirror waveguide structure designed to operateat a wavelength of 0.66 microns, and designed to provide a measurementand a reference for that measurement. The waveguide structure comprisesa sample 51 provided as a sensing layer on top of a thin layer ofsilicon nitride 52 (80 nm), which in turn is located on top of a thicklayer of silica 53 (500 nm). The silica layer is located on a secondthin layer of silicon nitride 54 (100 nm), which is located on a secondthick silica layer 55 (500 nm). The entire structure is located on asubstrate 56 (the undersides of which are angled to form a prism). Theillustrated waveguide structure essentially comprises a first resonantmirror structure located on top of a second resonant mirror structure.The sensing layer may be low index dextran gel, or may be any othermedium capable of supporting biological or chemical interactions, forexample sample separation, antibody-antigen interactions, etc.

An optical sensor according to the invention is shown in FIG. 31. Thesensor is similar to existing apparatus which is used to performresonant mirror measurements and surface plasmon resonance measurementsbut modified to incorporate the waveguide structure of FIG. 30. Theapparatus according to the invention comprises a light source 57 whichproduces a beam of light at a known wavelength. A polariser 58 isarranged to provide equal proportions of TE and TM excitation, and alens 59 focuses the beam to a fan-shape. The beam is directed into aprism 56 which forms part of the waveguide. Although the prism 56 isshown as being triangular, it could be of any suitable shape (forexample rectangular), and other forms of substrate may be used. Becausethe beam is directed towards the waveguide as a fan, light is incidentat the waveguide from a range of different angles. The incident lightwill either be coupled to two resonant mirror modes centred respectivelyon the layers of silicon nitride 52, 54 or will be reflected from thewaveguide structure without being coupled to a resonant mirror mode.Light will be coupled from the prism 56 of the waveguide in the form ofa fan, and, after passing through a collimating lens 60 and an analyser61 (comprising a polariser and a quarter-wave plate), will be incidentupon a detector 62. The detector 62 comprises an array of charge coupleddevices (CCD's) which detect the intensity of light at differentsections of the fan, i.e. at different incidence angles.

The phase of light coupled from the waveguide will undergo a full 2πchange on passing through a resonance peak (i.e. an angle of incidencewhich provides efficient coupling to the modes centred on the layers ofsilicon nitride 52, 54). It is the position of these phase changes whichis monitored to measure changes in the optical properties of the sample51. The resonant mirror optical modes for TE and TM excitation arewidely separated in incidence angle. As the angle of the incident lightapproaches the angle needed to excite, for example, a resonant TE modecentred on the first layer of silicon nitride 52, the phase of lightcoupled from the waveguide will be shifted, and will pass through amaximum phase shift of π at the resonance peak. Light which is coupledto a TM mode of the same layer at the same angle of incidence will notpass through a resonant mode. The polariser 60 is arranged to mix lightfrom the TE and TM modes, thereby providing interference which passesthrough a peak of intensity as the TE mode passes through resonance. Theposition of the peak of intensity is dependent upon the opticalproperties of the waveguide structure.

The CCD array of the optical sensor may be replaced by a pair ofphoto-diodes (not shown) mounted so as to be capable of translation in adirection perpendicular to the direction of the light reflected from thewaveguide. In use the photo-diodes would be positioned where peaks ofintensity occurred, and would be translated to follow the peaks ofintensity during an experiment, thereby allowing measurement of thedegree of movement of those peaks of intensities.

A It will be appreciated that the combination of the light source 57 andlens 59 of FIG. 31 may be replaced by a collimated light source, mountedon a swinging arm. The arm would be swung through a required range ofangles to produce illumination at the waveguide similar to the fan oflight shown in FIG. 31. The lens 59 would not be required by theswinging arm arrangement.

The lens 60 may be removed from the apparatus without significant lossof performance.

Since the waveguide structure comprises two resonant mirror waveguides,a first angle of incidence will excite a mode centred on the first layerof silicon nitride 52 (i.e. a mode of the first resonant mirror), and asecond angle of incidence will excite a mode centred on the second layerof silicon nitride 54 (i.e. a mode of the second resonant mirror).

FIGS. 32 and 33 illustrate optical fields associated with modes centredrespectively on the first and second silicon nitride layers (52, 54) ofthe waveguide structure of FIG. 30 (the reference numerals are as usedin FIG. 30). A significant fraction of the mode illustrated in FIG. 32extends into the sensing layer 51 of the structure, and this mode willtherefore be affected by changes of the optical properties of thesensing layer 51 (this mode will be referred to as the measurementmode). In contrast, only a very small proportion of the mode illustratedin FIG. 33 extends into the sensing layer 51, and this mode will belargely unaffected by changes of the optical properties of the sensinglayer 51 (this mode will be referred to as the reference mode).

Using the structure of FIG. 30 it is possible to differentiate betweenchanges in optical properties of the sensing layer 51, and unwantedoptical effects such as any variation of the wavelength of lightincident on the waveguide structure. For example, if the wavelength ofincident light was to change, the angles of incidence required to exciteboth the measurement and reference modes would be altered, and thepositions of the corresponding measurement and reference peaks detectedby the CCD camera would be altered. The change in position of thereference peak is determined, and subtracted from the position of themeasurement peak to remove the effect of the wavelength change from themeasurement. This is illustrated in the experimental result shown inFIG. 34. The peaks on the graph are the recorded positions of outputsfrom an optical sensor incorporating the waveguide structure shown inFIG. 30, for four different wavelengths of incident light. The peaks 63at the left-hand end of the graph are measurement peaks, and the peaks64 to the right-hand end of the graph are reference peaks. From FIG. 34,it is clear that both the measurement and reference modes (and thepositions of the corresponding peaks) are affected similarly by thechanges of wavelength.

In contrast, a change of the refractive index of the sample comprisingthe sensing layer 51 will significantly affect only the mode shown inFIG. 32. This is illustrated in FIG. 35, where the positions of themeasurement peaks 65 to the left-hand end of the graph vary as therefractive index of the sample comprising the sensing layer 51 changes,and the reference peaks 66 to the right-hand end of the graph aresubstantially unaffected.

The invention is advantageous because it removes the need forstabilisation of the wavelength of the incident light; the effect ofwavelength variation being removed by comparison of the measurement andreference peaks. A further advantage of the invention is thatmeasurements are unaffected by changes in the temperature of thewaveguide structure. If the temperature of the waveguide structure wasto change, this would affect the measurement and reference modesequally, and the effect of the temperature change would thus beeliminated by comparison of the measurement and reference peaks.

In the above description it has been assumed that effects which act onboth the measurement and reference modes (for example a change ofwavelength) will affect each mode equally. However, since the modes arenot identical, each mode will in fact behave slightly differently. Bycalibrating the effect of wavelength and temperature variations when thesample comprising the sensing layer 31 is inactive, the accuracy ofsubsequent measurements may be maximised. In the alternative, doping maybe introduced into the structure in such a manner as to ensure thatmeasurement and reference modes have the same behaviour with respect totemperature and wavelength variations.

The thickness of the first layer of silica 53 (in the waveguide shown inFIG. 30) is importance to the operation of the invention. If this layeris too wide then the measurement mode will not be excited, and if thelayer is too thin then the reference mode will extend too far into thesensing layer 51, and will not provide a reference substantiallyindependent of the sensing layer 51. The thickness of the first siliconnitride layer 52 may be chosen to be slightly less than the thickness ofthe second silicon nitride layer 54. This is so that the first layer isslightly more ‘leaky’ than the second, thereby ensuring that asufficient proportion of the measurement mode will penetrate into thesensing layer 51.

FIG. 36 illustrates an anti-resonant reflecting optical waveguide(ARROW) biosensor, which is capable of supporting a reference mode whichis unaffected by changes in the optical properties of a sample. TheARROW structure is comprised of the following layers:

Layer Reference Thickness No. Material Refractive Index (microns) 67Substrate (SF 10) 1.72038 Semi-infinite 68 Silica 1.47 0.55 69 SiliconNitride 2.00 0.08 70 High-Index Layer 1.65 3.00 71 Silica 1.47 0.55 72Silicon Nitride 2.00 0.08 73 Dextran Gel 1.35 3.00 74 Water 1.333Semi-infinite

It will be understood that the above materials and thicknesses are givenonly as examples, and other materials of appropriate thicknesses may beused to construct a waveguide capable of supporting ARROW modes. Inparticular, the dextran gel is just one of many possible materials whichmay be used to support a sample of interest. The layer of substrate maybe considered to be semi-infinite, and the substrate is shown as being 1micron thick to allow a zero position to be defined.

FIGS. 37 and 38 respectively show first and second ARROW modes centredon layers 73 and 70 of the structure of FIG. 36. The modes are excitedby directing incident light from an appropriate angle, in a manneranalogous to that described above. The physics of ARROW waveguides iswell known, and is described in the paper: Duguay el al, Appl. Phys.Lett., 49 (1986) 13-15. The reference mode of the ARROW waveguide (i.e.the second mode), as shown in FIG. 38, may be used to eliminate unwantedartifacts from a measurement of the optical properties of a sample, inthe manner described above.

It will be understood by those skilled in the art that the waveguidestructure illustrated in FIG. 36 may also be made to support resonantmirror modes. The silica layers 68, 71 are included in the waveguide toallow the resonant mirror modes to be supported. This particularwaveguide is designed to support resonant mirror modes at a wavelengthof 0.66 microns. A waveguide constructed without these layers would notsupport resonant mirror modes, but would still be capable of supportingARROW modes.

FIG. 39 illustrates a further embodiment of the invention. A waveguidecomprises a prism 75, a layer of silica 76 and a high index siliconnitride layer 77. A layer of chemically inert material 78 with arefractive index lower than that of the silicon nitride layer 77 islocated at a left hand end of an upper surface of the silicon nitridelayer 77, and a sample 79 is disposed as a sensing layer so as to coverthe remaining upper surface of the silicon nitride layer 77 and an uppersurface of the polymer layer 78.

In use, when a fan of light is coupled into the waveguide of FIG. 39, amode centred on the silicon nitride layer 77 will be excited. At a lefthand end of the waveguide the mode will extend into the polymer layer78, but will not extend substantially beyond the polymer layer 78. At aright hand end of the waveguide the mode will extend into the sensinglayer 79. Thus, the excited mode comprises two components which may beconsidered to be two modes, the left hand mode being substantiallyunaffected by changes of the properties of the sample (i.e. a referencemode) and the right hand mode being sensitive to changes of theproperties of the sample (i.e. a measurement mode). The waveguide shownin FIG. 39 may be used with the apparatus shown in FIG. 31, as describedabove.

It will be understood that the layer of silicon nitride 77 could bereplaced by a layer of any suitable material having a refractive indexgreater than that of the silica 76, the inert material 78 and thesensing layer 79. Similarly, the layer of silica may be replaced by anyother suitable material. The chemically inert material 78 may besilicon, or alternatively a polymer chosen because it does not react toan analyte to be monitored may be used.

FIG. 40 shows a first resonant mirror mode located at a right hand endof the waveguide structure of FIG. 39. A substantial proportion of thefirst mode extends into the sensing layer 79, and this mode willtherefore be sensitive to changes of the optical properties of thatlayer (i.e. the first mode is a measurement mode).

FIG. 41 shows a second resonant mirror mode located at a left hand endof the waveguide structure of FIG. 39. Only a very slight proportion ofthe second mode extends into the layer of sensing material 79, and thismode will therefore be substantially unaffected by changes of theoptical properties of that layer (i.e. the second mode is a referencemode).

The waveguide of FIG. 39 is advantageous over previously describedembodiments because it has fewer layers and is therefore easier andcheaper to fabricate.

It is noted that the substrate of the above waveguide structuresdescribed in FIGS. 30, 36 and 39 (i.e. layers 56, 67 and 75) may beangled to form a prism, or may be provided with a planar base which isto be located upon a separate prism made from the same material as thesubstrate.

Each of the waveguide structures described in FIGS. 30, 36 and 39 mayalso be provided in what is known as a symmetric form. In each case thesymmetric form comprises a central layer of sensing medium, withidentical layers disposed on either side of the central layer, thelayers on both sides being arranged as shown in FIGS. 30, 36 and 39.Light coupled to a symmetric waveguide will undergo reflection asdescribed above, but will also be transmitted by that waveguide when aresonant mirror mode or ARROW mode is excited. The proportion of lightreflected by the waveguide when a resonant mirror mode or ARROW mode isexcited is reduced as a consequence of the transmission, and thepresence of such a mode will therefore indicated by a dip in theintensity of light reflected by the waveguide. Symmetric arrangements ofwaveguides are thus advantageous because they allow the detection ofresonant mirror modes or ARROW modes without the use of polarisers orwave-plates.

Detection of resonant mirror modes or ARROW modes may also be achievedby introducing absorption or scattering loss into the layer of sensingmedium of the waveguide structures shown in FIG. 30, 36 or 39. In awaveguide of this type excitation of a resonant mirror mode or ARROWmode will lead to a reduction of the intensity of light reflected fromthe waveguide, due to losses occurring in the sensing medium. Thepresence of a resonant mirror mode or an ARROW mode will thus beindicated by a dip in the intensity of reflected light. The introductionof loss into the sensing medium is advantageous because it simplifiesdetection, as discussed above in relation to the symmetric waveguideform. It is noted that the presence of loss in the sensing medium willincrease the range of incidence angles which are capable of exciting aresonant mirror mode or ARROW mode, thereby reducing experimentalsensitivity.

The sensor waveguides described in relation to FIGS. 30 to 41 aresimilar in terms of dimensions and optical properties to sensorwaveguides used in existing resonant mirror optical biosensingapparatus. A sensor waveguide as descried may therefore be introducedinto existing biosensing apparatus with a minimal amount ofmodification. The temperature stabilisation and current stabilisationused by existing biosensing apparatus is not required when sensorsaccording to the invention are used, and may be dispensed with, therebyreducing the complexity and cost of the apparatus.

It will be understood that the structures described in relation to FIGS.30, 36 and 39 are concerned with providing waveguide structures havingreference modes which are unaffected by changes of optical properties ofa sample. The invention may be applied to other waveguide structures,and such applications will be apparent to those skilled in the art.

What is claimed is:
 1. An optical sensor comprising a waveguide having asubstrate, a layer of metal or metal alloy disposed on top of thesubstrate, and a medium disposed as a sensing layer on top of the layerof metal or metal alloy, the medium having optical properties whichchange if the medium is exposed to conditions to be sensed, the sensorfurther comprising means for directing light towards the layer of metalor metal alloy through the substrate over a range of incident angles,and detection means for detecting the intensity of light returned fromthe waveguide over a range of detection angles, the means for directinglight being configured to direct light such that a leaky waveguide modeis excited within the sensing layer, and the means for detecting theintensity of light being arranged to detect variations with detectionangle in the intensity of returned light resulting from the excitationof the leaky waveguide mode; characterised in that the waveguide isconfigured such that the overlap of the optical field with the layer ofmetal or metal alloy is less for light incident at an angle whichresults in excitation of a leaky waveguide mode than for light incidentat an angle which does not result in excitation of a leaky waveguidemode, whereby the detected intensity peaks at a detection angle relatedto an incident angle which results in excitation of a leaky waveguidemode.
 2. An optical sensor according to claim 1, wherein the substratecomprises a prism or grating for coupling light into the waveguide mode.3. An optical sensor according to claim 1, further comprising a broadband optical source.
 4. An optical sensor according to claim 3, whereinthe optical source is a light emitting diode.
 5. An optical sensoraccording to claim 1, wherein the detection means is acharge-coupled-device array (CCD) comprising cells of sufficiently smalldimensions to allow resolution of the intensity variations resultingfrom the excitation of the waveguide mode.
 6. An optical sensoraccording to claim 1, wherein the detection means is a singlephoto-diode arranged to be translatable across the light returned fromthe waveguide.
 7. An optical sensor according to claim 1, wherein thethickness of the layer of medium is greater than 200 nanometers.
 8. Anoptical sensor according to claim 1, wherein the thickness of the layerof medium is greater than 300 nanometers.
 9. A method of optical sensingcomprising providing a waveguide comprising a substrate, a layer ofmetal or metal alloy disposed on top of the substrate, a medium disposedas a sensing layer on top of the layer of metal or metal alloy, themedium having optical properties which change if the medium is exposedto conditions to be sensed, directing light towards the layer of metalor metal alloy through the substrate over a range of incident angles,and detecting the intensity of light returned from the waveguide over arange of angles, wherein the incident light is directed such that awaveguide mode is excited within the sensing layer, and variations inthe intensity of returned light resulting from the excitation of thewaveguide mode are detected; characterised in that the waveguide isconfigured such that the overlap of the optical field with the layer ofmetal or metal alloy is less for light incident at an angle whichresults in excitation of a leaky waveguide mode than for light incidentat an angle which does not result in excitation of a leaky waveguidemode, whereby the detected intensity peaks at a detection angle relatedto an incident angle which results in excitation of a leaky waveguidemode.